Diesel particulate filters (DPFs) and the like are conventionally used for removal of particulate matter (PM) contained in exhaust gases discharged from internal combustion engines, particularly diesel engines. The combustion temperature of PM is as high as 550° C. to 650° C., so that high temperature is required to burn PM. Therefore, as a porous material used in a DPF and the like, a material is desired which has low thermal expansivity and superior thermal shock resistance to avoid the occurrence of fatigue due to thermal shock within the operating temperature range. Aluminum titanate meets these performance demands and has a high melting point. For this reason, aluminum titanate has been expected as a porous material used such as for a DPF and developed in various ways.
Meanwhile, since the combustion of PM requires high temperature, attention is being paid to a method in which a catalyst is supported on a DPF to steadily combust PM at low temperature. The supporting of the catalyst is performed, after the production of the DPF, by impregnating the DPF with a solution containing catalyst particles or applying the solution to the DPF, so that a catalyst layer is formed on the wall surfaces of the DPF and in the pores of the DPF walls. However, there arises a problem in that some of the pores are clogged by the catalyst layer so that pores having been previously interconnected are isolated from one another to reduce the number of exhaust gas flow channels, resulting in reduced PM combustion efficiency.
Patent Literature 1 proposes that in order to provide a sintered aluminum titanate body having not only high strength without impairing high melting point and low thermal expansivity characteristics possessed by aluminum titanate but also less degradation in mechanical strength due to repeated thermal history, a substance formed by adding magnesium oxide and silicon oxide to aluminum titanate is sintered.
Patent Literature 2 discloses the production of an exhaust gas filter using columnar aluminum titanate and proposes to produce the exhaust gas filter in which, when the columnar aluminum titanate particles have a negative coefficient of thermal expansion in the longitudinal direction, they have a positive coefficient of thermal expansion in the direction perpendicular to the longitudinal direction or in which, when the columnar particles have a positive coefficient of thermal expansion in the longitudinal direction, they have a negative coefficient of thermal expansion in the direction perpendicular to the longitudinal direction. However, the literature does not disclose a specific method for manufacturing columnar aluminum titanate. Furthermore, the literature also does not disclose specific shape features of the columnar aluminum titanate, such as the aspect ratio of the columnar shape.
To solve the problem of reduced PM combustion efficiency due to the formation of a catalyst layer on the DPF wall surfaces and in the pores of the DPF walls, Patent Literature 3 proposes a method in which a spherical pore-forming agent, a columnar pore-forming agent, and an inorganic binder are added in respective predetermined amounts to a catalyst and mixed to form a slurry and a support is immersed into the slurry. However, there arises a problem in that the catalyst may enter microcracks in the DPF to increase the coefficient of thermal expansion. There also arises a problem of poor production efficiency of the catalyst-supported DPF.